Gravity and density gradiometer for boreholes



Nov. 18, 1952 F. s. BQUCHER 2,618,156

GRAVITY AND DENSITY GRADIOMETER FOR BOREHOLES Filed Dec. :51, 1949 4 Sheets-Sheet 1 Fla-1 1 1110.14. GJbOUChQF' Inventor abs abhor-neg Nov. 18, 1952 F. G. BOUCHER GRAVITY AND DENSITY GRADIOMETER FOR BOREHOLES 4 Sheets-Sheet 3 Filed Dec. 31, 1949 Frail-4. G. 23:06]). er 'Jrzvepbor 4 Sheets-Sheet 4 1 111121 G boudhar' Unvgnbor b551 1 Quarries dotqdu 290 F e BOYUCHER cmwrrv mu DENSITY GRADIOMETER FOR BOREHOLES Nov. 18, 1952 Filed Dec 31 1949 Patented Nov. 18, 1952 GRAVITY AND DENSITY GRADIOMETER FOR BOREHOLES Frank G. Boucher, Tulsa, Okla., assignor to Standard Oil Development Company, a corporation of Delaware Application December 31, 1949, Serial No. 136,314

Claims.

The present invention is concerned with an improved apparatus for determining the density of substrat surrounding a bore hole. More particularly the invention is concerned with the provision of, a sensitive device capable of being lowered into a bore hole to measure gravity gradients existing therein, from which gravity gradients differences in the density of the various strata traversed by the bore hole may be determined.

In the constant search ior'new sources of crude oil many means have been devised for determining whether any possibility of obtaining oil from a particular locality exists but all the methods involve considerable expenditure of time and effort and it is therefore desirable to obtain every bit of information that is available when an area is being explored. One valuable source of information regarding the geological structure of a particular area is the bore hole obtained when an oil well is being drilled. Not only is the material removed during drillin examined closely but the well bore itself is usually subjected to intensive examination to determine the nature and position of the various strata encountered. The numerous methods employed in such examination are referred to broadly as well logging methods. It is one of the objects of the present invention to provide a novel form of apparatus for well logging, particularly for logging gravity gradients existing in the bore hole.

As already mentioned it is possible to determine from the measured gravity gradients difierences in density between various strata. This serves not only as an aid to tying in geological information between wells in the same oil field but also is of value in determining whether various strata are compacted by virtue of the pressure exerted by the weight of overlying strata, since the density of the stratum determined in the bore hole can be compared with the density oi rock fragments from the same stratum after recovery from the bore hole. Similarly. the density of limestone beds can be determined and used as a measure of their porosity.

Density determinations of the layers near the earths surface are also of value for making more intelligent correction of data obtained in the usual gravity meter surveys wherein the gravity meter readings must be adjusted for the near surface density of the earth in the locality of the survey. Thus it is one object of this inven- 2 ing devices the gravity gradients that exist in the bore hole. Other objectives as well as the nature of the invention and the manner in which it is to be performed will be apparent from the ensuing description and from the drawing in which:

Fig. 1 is a schematic sketch showing one arrangement for suspending the device of this invention in a bore hole;

Fig. 2 is a schematic sketch showing an alternative arrangement for suspending the device;

Fig. 3 is a vertical perspective view oi the gravity gradient device contained within case ii of Fig. 1;

Fig. 4 is a schematic representation 01' the remotely operable clamping devices of Fig. 3;

Fig. 5 is a detail of a portion of-Fig. 3 together with associated elements and an electrical circuit, providing one means of restoring the mass suspension beam to a null position;

Fig. 6 is a schematic diagram of an alternative means for balancing the mass suspension beam; and

Fig. 7 is a schematic diagram showing a means for directly reading the amount of mass suspension beam unbalance.

Briefly, the present invention comprises a device consisting of a pair of weights of equal mass, shape and volume suspended from opposite ends of a movable beam, the two weights being spaced vertically from each other on suitable supporting means, e. g. wires. Attached to each supporting means is an additional weight of the same shape and volume as the first pair of weights but of much less mass, the two lighter weigh-ts being of the same mass, one of them being placed directly opposite each of the aforementioned weights of heavy mass. In order that the device may be lowered into a bore hole a suitable case is provided, together with means for remotely clamping and unclamping the movable beam and the weights, means for remotely restoring the balance of the movable beam when it becomes unbalanced due to gravity gradients. means for remotely determining when the beam is in balance and means for measuring the force required to restore the beam to balance.

Referring now specifically to Fig. 1 of the drawing, an outer case 2 is suspended in a bore hole ill! by means of a cable I which is provided with suitable electrical conductors for use in conducting current between the device and suitable detecting and operating instruments at the earth's surface. An'inner casing 4 is suspended within outer case 2 by suspension means 3 which may be,

of the cover of case 4.

forexample, a single wire attached to the center Case 4 is sufllciently smaller in diameter than case 2 so as to permit as much as 10 tipping of case 2 from the vertical while case 4 remains vertical. Space 5 between the two cases is preferably filled with a liquid such as water or oil or the like.

An alternative means for suspending case 4 within case 2 is shown schematically in Fig. 2 and consists simply of a gimbal ring 8 pivotally attached to case 2 on pivots I and pivotally supporting case 4 on pivots la.

The operating mechanism contained within case 4 is shown in Fig. 3, this being a verticalperspective view in which portions of the supporting structure have been omitted in the interest of clarity. Placed within case 4 is a suspending bracket 9 from which a suspending beam I is hung on supporting wires II which may be suitably held in place by means of clamping blocks I2. Attached to the center of beam I0 and placed in perpendicular relation thereto in a horizontal plane is a mass suspension beam I to which are attached mass suspension wires 34 and 35. A weight of fairly heavy mass 38 is attached to wire 34 and a weight 4| of equal mass, shape and volume is similarly attached to wire 35 but at a considerably greater distance below beam I 5 than weight 38. Also attached to wire 34 at the same vertical distance from beam I5 as weight 4| is a lighter weight 39 having the. same shape and volume as weights 38 and 4|. In the same manner likewise, weight 40 of the same shape and volume as the other three weights but of a mass equal to weight 39 is attached to wire 35 at the same vertical distance from beam I5 as weight 38. Weights 38 and 4| are made from a material of high density such as lead or gold and weights 39 and 40 are made from lighter materials such as magnesium or aluminum or alloys thereof. Wires 34 and 35 are preferably made of material having very low ductility, high tensile strength and a low coeflicient of thermal expansion. For example they may be made of tungsten or of a beryllium-copper alloy or other suitable alloy, and will be of as small a diameter as practical, i. e. in the range of about 0.001 to 0.003 inch.

Attached to beam III by means of vertical threaded rods I4 are additional labilizing weights IB and I9, whose purpose is to adjust the period of swing of beam I5 about the axis of beam ID to the desired value. An additional weight 20, suspended'below beam I0 on threaded rod I4a, can be provided if desired to raise or lower the center of gravity of the beam system.

Also flxed to beam I0 are upright posts I6 which carry a thin wire I! passing directly over the intersection of beams I0 and I5. The purpose of wire I1 is to indicate the null position of beam I5 by optical means as will be explained in more detail in connection with Fig. 5. It may be mentioned here briefly, however, that mirror 23 is adapted to receive a light beam from the lens and prism system 2I and 22 and to direct it past wire I'I into lens 24, a suitable light source and optical measuring means. being contained within case 4 above bracket 9.

The sensitivity of the instrument will be found to be more or less proportional to the vertical distance between weights 38 and 4| and to the square of the period of swing of beam I5. For practical purposes the beam I5 should have a period of about 20 to 30 seconds, preferably about 25 seconds, and the vertical distance between weights 38 and 4| should be of the order of 8 to 4 12 feet. It is to be understood, of course, that the invention is not to be limited by these specific dimensions, and greater distances between the weights may be employed if desired.

In order that the sensitivity of the instrument will not be adversely affected by damage to the weights and/or supporting means when the apparatus is being lowered into or raised out of the bore hole, remotely operable clamping means are provided for holding the beams and weights in place when desired. The clamping means for the weights comprises a horizontal arm 46 slidably supported on vertical posts 48 and adapted to be raised or lowered by threaded rod 49. Attached to arm 46 is a cross-arm 41 to which are fixed n locking posts 50, adapted to press each of the weights up against a clamping block 43. Only one block 43 is shown, in phantom view, but a similar block is provided for each of the four weights. Blocks 43 and posts 48 are supported by bracket 42, fixed to case 4. The method of raising and lowering arms 46 and 41 will be more readily apparent from the ensuing description in connection with Fig. 4. Similar clamping means are provided for the lower weights 39 and 4|, but these are omitted from the drawing in Order to prevent undue complication thereof. Each of the clamping blocks 43 also supports an air damping cup 44. These cups surround the weights 38 to 4| inclusive, but do not touch them and thus serve to damp the moving system of beam and weights without interfering with its motion or its sensitivity.

To preserve the sensitivity of the instrument, springs 36 may be attached to suspension wires 34 and 35 at points intermediate each of the clamping devices. These springs are of such strength that they will not give appreciably under the load of the weights and yet will take up the slack in the suspension wires when the weights are lifted into clamping position, thus preventing kinks in the suspension wires.

Similar springs 36 may also be attached intermediate the ends of wires II for the same reason. An embodiment of the invention in which the springs 36 are omitted is also contemplated, their use being advocated only when extreme sensitivity is desired.

Means for locking beam I5 in place comprises a fixed bracket 53 placed just above beam I5 and a movable arm 54 pivotally attached below beam I5 on a vertical extension of bracket 53. The free end of arm 54 is adapted to be raised or lowered by means of vertical rod 56 having a threaded end which engages a tapped motor shaft 51. A similar locking device is provided on the other end of beam I5 but is omitted from the drawing.

The operation of the clamping devices will be readily understood by reference to Fig. 4. Threaded rods 49 and tapped shaft 51 are adapted to be rotated in either direction by means of gear-reduced reversible electric motors 60, 6| and 62, which are suitably fixed within case 4. These motors can be remotely operated in either direction from the'earth's surface through suitable conductors within cable I. The electrical leads supplying each motor are not shown in the drawing but their distribution within case 4 is merely a matter of convenience and design. In order that the motors will not exert too great a force on the clamping devices each motor may be provided with a friction clutch. Slipping of the clutch can be detected at the surface by means of an ammeter in the power circult. A related arrangement of remotely operable clamping means is described and claimed in depending application Serial #794,617 of Herbert E. Owen, filed December 30, 1947, now U. S. Patent 2,560,366, issued July 10, 1951.

To ensure that there will be no adhesion of beam IE to bracket 53 or to arm 54, adjustable screw clamping points and wire springs may be provided on the bracket and arm in the manner described in U. s. Patent 2,349,404.01 Donald W. Blair and Frank G. Boucher.

Briefly the device of this invention operates on the following principle: Neglecting lighter weights and 40 for the moment, it is apparent that it weights 38 and 4| were placed at the same elevation the beam l would be in balance. Since the latter two weights are at different elevations, however, any gravity gradient in the vicinity oi the apparatus will cause beam IE to be unbalanced, the amount of unbalance being proportional to the gravity gradient. The purpose of weights 39 and 40 is to compensatefor possible difierences in air density at the upper and lower weight levels resulting from temperature differences, for example.

In order to measure gravity gradients as a function of the amount of unbalance of the beam, several arrangements may be employed, one of which is shown in Fig. 5. Light from a suitable light source 30 is focused by means of lens arrangement it on to a reflecting prism 22, thence to reflecting mirror 23, which is fixed between posts it by means of a bracket ill which may be attached to case t at any suitable point as, for example, to bracket 9. Reflected light passes from mirror it past wire ill and through lens 2 3 to. a beam-splittin prism 25. One half of the split beam is directed toward photosensitive cell it and the other half toward photosensitive cell ill. Lens at has the proper magnifying characteristics to enlarge the shadow of wire ll so that it will cover a considerable portion of the bottom areas of reflecting prism 25. When beam i5 is in a level position the amounts of light falling on each half of prism 25 will be equal. Cells it and M are connected in electrical opposition and are also connected to a balanced amplifier 28, the output of which is sent to the earths surface through lead it which may be suitably carried by cable ll. Lead 29 is connected at the earths surface with a galvanometer 33. When the same amount of light reaches cells 2t and El the net electrical output will be zero and hence galvanometer 33 will give a zero reading. When beam it is ofl balance more light will reach one cell than the other and galvanometer it will read either plus or minus.

When beam is is indicated to be ofi balance as a result of a gravity gradient its balance may be restored by one of several means. For example, a coil l2 may be carried on an extension ll of beam i5 and the coil may be arranged in the field of a suitably supported cup-shaped magnet it having a central core M. The central core constitutes one'pole of the magnet and the outer rim oi the cup the other pole. An electrical current may be fed through coil 12 by means of leads l5 and 16. Current reading means may comprise, for example, a sensitive potentiometer and galvanometer arrangement. By a suitable calibration the amount of current necessary to bring beam l5 back to balance may be determined as a function of difference in gravity forces; In order to avoid reversing the current through coil 12 to take care of unbalances of beam IS in both directions, the beam may be designed so as to be off balance in a direction opposing the force of the magnetic field and cur rents detected by measuring means 80 below a stated value may be taken as a. measure of beam unbalance in one direction and those above that value as a measure of beam unbalance in the opposite direction.

An alternative means of balancing beam I5 is depicted diagrammatically in Fig. 6. One end of helical spring BI is attached to beam 15 at a point near its fulcrum. The other end of spring 8! is attached to a micrometer head 82 mounted on the shaft of a selsyn motor 85 suitably supported within case 4. Leads 86, 81 and 88 connect motor 85 with a selsyn generator 90 at the earths surface, alternating current being fed to the selsyn system from a suitable source 9|. Thus selsyn motor 85 may be turned a desired number of revolutions by selsyn generator 90 and the number of turns needed may be calibrated as a function of the amount of force required to be exerted on spring iii to bring beam 55 back into balance. The turns made .by selsyn generator 90 may be recorded by a counter 93 and Vernier arrangement t2, the vernier also constituting a hand wheel for turning the selsyn generator.

Instead of providing for a null readin arrange ment for beam it as described in connection with Fig. 5, provision may be made for a direct reading of the amount of beam unbalance as shown, for example, in Fig. 7. Wire H is replaced by a relatively wide strip er whose shadow is 'focused by means of lens arrangement it on to a photoelectric cell 953, the relation of the elements being such that when beam is is off balance in one direction no shadow will be cast by strip ill on cell as and when beam i5 is off balance in the opposite direction strip 9 will cast a full shadow. Thus suitable calibration can be made between the swing of beam i5 and the amount of light detected by cell 98.

Calibration of the completely assembled instru ment may be made by comparing the amount of beam displacement obtained when the instrument is surrounded by various media of known density. a

It is intended that this invention be limited only by the following claims and not by the specific embodiments described in the foregoing specification.

What is claimed is:

1. An apparatus for determining gravity gradients in a borehole comprising a. case adapted to be lowered into a borehole, a first suspending beam within said case, a pair of supporting wires attached to said case in the interior thereof and supportin said suspending beam in a horizontal position, a mass suspension beam attached to the center of said first suspending beam and at right angles thereto in a horizontal plane, a pair of mass suspension wires attached to the ends of said mass suspension beam and extending downwardly therefrom, a first relatively heavy mass attached to one of said mass suspension wires at a point intermediate its ends, a first relatively light mass attached to the lower end of said last named wire, a second relatively heavy mass attached to the lower end of the second of said mass suspension wires at a vertical distance from said mass suspension beam exactly equal to the vertical distance of said first relatively light mass from said beam, a second relatively light mass attached to said second mass suspension wire at a point intermediate its ends and at a vertical distance from said mass suspension beam exactly equal to the vertical distance of said first relatively heavy mass from said beam, said pair of relatively heavy masses being of the same weight, size and shape, said pair of relatively light masses being of equal weight and of the same size and shape as said relatively heavy masses, and means adapted to detect remotely a state of unbalance of said mass suspension beam.

2. Apparatus according to claim 1 including helical springs fixed in each of said supporting wires intermediate the ends thereof.

3. Apparatus according to claim 1 including helical springs fixed in each of said mass suspension wires at points intermediate said masses and at points intermediate said upper masses and said mass suspension beam.

4. Apparatus according to claim 1 including remotely operable clamping means adapted to clamp at least one of said masses against movement thereof, said clamping means comprising a block held by said case and having at least a portion projecting over said mass but normally out of contact therewith, a lifting member disposed below said mass, vertical guide means supported by said case and slidably receiving said lifting member, lifting means attached to and supporting said lifting member, and remotely operable motor means selectively driving said lifting means so as to raise said lifting member, thereby pressin said mass against the projecting portion of said block, and to lower said lifting member, thereby moving said mass out of contact with said block and moving said lifting member out of contact with said mass.

5. Apparatus according to claim 1 in which said case is pivotally supported adjacent its upper end within an outer protective case, the space defined between the two cases being filled with a viscous liquid.

6. Apparatus according to claim 1 in which said means for detecting unbalance of said mass suspension beam comprises an elongate member of finite width held by said first suspended beam above the point of intersection of said beams and parallel to said first suspending beam, means adapted to cast a shadow from said elongate member and photoelectric means adapted to respend in relation to the position of said shadow. 7. Apparatus according to claim 6 in which said elongate member comprises a fiat strip.

8. Apparatus according to claim 1 in which said means for detecting unbalance of said mass suspension beam comprises a wire held by said first suspended beam above the point of intersection of said beams and parallel to said first suspending beam, means adapted to cast a shadow from said last named wire and photoelectric means adapted to respond in relation to the position of said shadow.

9. Apparatus according to claim 1 including means for remotely restoring said mass suspension beam to a state of balance in opposition to gravity gradients causing estate of unbalance thereof, said balance restoring means comprising a coil of wire attached to one end of said mass suspension beam, a magnet disposed adjacent said coil, a source of current connected to said coil and means for indicating the amount 01' current supplied to said coil.

10. Apparatus according to claim 1 including means for remotely restoring said mass suspension beam to a state of balance in opposition to gravity gradients causing a state of unbalance thereof, said balance restoring means comprising a helical spring attached at one end to said mass suspensionbeam at a point adjacent the fulcrum of said beam, a selsyn motor adjacent said helical spring, means connecting the free end of said helical spring to said selsyn motor. a selsyn generator at the earth's surface, an electrical circuit connecting said generator with said motor. a source of electrical current for said generator, and means for measuring the revolutions made by said generator.

FRANK G. BOUCHER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,898,534 Hartley Feb. 21, 1933 2,149,953 Birnbaum Mar. '7, 1939 2,239,049 Morris Apr. 22. 1941 2,294,201 Pepper Aug. 25, 1942 2.296.330 Blau Sept. 22, 1942 2,327,697 Boucher Aug. 24, 1943 2,367,126 James Jan. 9, 1945 2,384,739 Hasbrook Sept. 11, 19% 

